This invention relates to diagnostic x-ray apparatus generally and, in particular, to a system for controlling an x-ray tube to emit pulses of x-rays of different average energies in rapid sequence.
An occasion for using a sequence of x-ray pulses at different energies is when it is desirable to selectively suppress or enhance the contribution to an x-ray image from a certain material. One instance is the imaging of body structures containing small amounts of an x-ray opaque dye, for example an iodine compound. Applications include the imaging of blood vessels opacified by intravenous injections of dye, or the imaging of tumors and/or organs slightly opacified by a dye. In normal x-ray images, the low image contrast of the iodine-containing region can be masked by overlying and underlying bone or soft tissue structures and be made very difficult to see. Energy subtraction, as the method of combining images at different x-ray energies is called, can enhance the contrast of one material, for example iodine, with respect to that of other materials, for example bone and/or soft tissue, and greatly improve the visibility of the structures of interest. Another application of energy subtraction may be in separating the contributions to an image due to bony structures from those due to soft tissue structures, for example in chest imaging. A sequence of x-ray pulses at different energies can also be used in computerized tomographic (CT) imaging. In CT, the x-rays of different energies can be used to provide information on the chemical composition as well as density of a transverse section. The use of pulses at different energies also has the benefit of reducing certain beam-hardening artifacts in CT.
An alternative to energy subtraction for the improved visualization of administered contrast agents is temporal subtraction. In temporal subtraction images taken before and after the injection of the iodinated dye are subtracted. The basic limitation of temporal subtraction is that the images being combined are separated in time by several seconds, and any motion that occurred between the acquisition of the two images will result in mis-registration artifacts in the image. Further, temporal subtraction is not well suited to imaging contrast-producing materials that are slowly or not at all time varying, for example, bones or iodine dye sequestered by tumors. However, if the images for energy subtraction are gathered in rapid time sequence little or no motion could have occurred during the image acquisition time and subtraction images with no mis-registration artifacts can be produced. What is required, then, is a means by which the average photon energy of the x-ray beam can be switched very quickly.
The computational methods and theoretical aspects of energy subtraction are described in the literature. Theories for energy subtraction using two or three different x-ray energies have been developed. This application describes in detail an improved method for producing x-ray pulses of two energies in rapid sequence. It is understood that the same concept could be used to produce pulses with three different x-ray spectra in rapid sequence.
The average energy of an x-ray beam can be affected by added x-ray filtration, and multi-energy methods using switching filters have been described. However, in order to change the x-ray energy significantly, heavy filtration is required and the resulting x-ray intensity is greatly reduced. A preferred method is to change both the x-ray filtration as well as the voltage at which the x-rays are produced.
Ideally, the required x-ray switching system should rapidly change both the x-ray tube peak kilovoltage (kVp) as well as the tube current (mA). It is preferred to have a higher mA for the lower kVp pulses. Prior art switching circuits which change the kVp only through the use of a variable high voltage control, such as a high voltage tetrode, result in an mA change in the wrong direction. That is, the lower the kVp applied to the tube, the lower the tube current. If the system is set up so that the highest kVp pulse has a current that does not exceed the tube rating, the lower kVp pulses with the lower current will likely produce a less than optimum total intensity. Changing mA in the traditional manner, that is, by changing x-ray tube filament current, is too slow due to the thermal lag of the filament.